Ink jet printers have print heads that operate a plurality of ejection jets from which liquid ink is expelled. The ink may be stored in reservoirs located within cartridges installed in the printer, or the ink may be provided in a solid form and then melted to generate liquid ink for printing. In these solid ink printers, the solid ink may be in either pellets or ink sticks. The solid ink pellets or ink sticks are typically placed in an “ink loader” that is adjacent to a feed chute or channel. A feed mechanism moves the solid ink sticks from the ink loader into the feed channel and then urges the ink sticks through the feed channel to a heater assembly where the ink is melted. In some solid ink printers, gravity pulls solid ink sticks through the feed channel to the heater assembly. Typically, a heater plate (“melt plate”) in the heater assembly melts the solid ink impinging on it into a liquid that is delivered to a print head for jetting onto a recording medium.
After the ink is provided to the print head, it is ejected from print head orifices across an open gap to a receiving member to form an image. The receiving image member may be a revolving print drum or other intermediate offset member, such as a rotating belt. The image generated on the offset member is transferred to media that comes into contact with the rotating member. To facilitate the image transfer process, a pressure roller, sometimes called a transfix or transfer member, presses the media against the print drum. Specifically, the leading edge of the media is fed into a nip between the intermediate member and a transfix member so the two rotating members push the media through the nip for the transfer of the image from the intermediate member to the media. Offset printing refers to a process, such as the one just described, of generating an ink or toner image on an intermediate member and then transferring the image onto some recording media or another member.
In some offset printers, a release agent system is used to facilitate the transfer of the image from the offset image member to the media. The release agent system typically includes a release agent source and a release agent applicator. The release agent is typically a silicone oil or the like that may be applied to the rotating image member to produce a thin layer of release agent on the member. This layer reduces the adhesion of the ink to the image member. This reduction facilitates the transfer of the image to the media. Regulating the amount of release agent applied to an image member is important as too much release agent results in smeared images or oil stains on the media. Consequently, many release agent systems include a metering member that acts as a squeegee to remove excess release agent from an image member. Level sensors may also be used in the release agent source to detect a near exhaustion condition for the release agent source. Actually measuring release agent amount or level on an image member, however, is not known.
Although release agent is not applied to an image member in a toner printer, many toner printers include release agent systems. In toner printers, a rotating image member is typically a photoreceptor belt or drum. A light source is controlled with image data to remove charge from the photoreceptor belt to form a latent image of a document or the like. The latent image is then developed in a development station where a toner cloud is generated and toner is attracted to the latent image. The toner image is then transferred to media by bringing the media into contact with the toner image and applying charge to the back side of the media. The toner is attracted to the media. The media then moves to a fuser station where the media passes through a pair of heated rollers to fuse the image to the media. These heated rollers are frequently called fuser rolls.
Because heated toner displays some degree of tackiness, the heated toner may attempt to remain in contact with one of the fuser rolls and impede the progress of the media along its feed path to a discharge area. To facilitate separation of media from a fuser roll, release agent may be applied by a release agent system to one or both fuser rolls. The layer of release agent helps reduce the adhesion of the media to a fuser roll so the media separates from the fuser roll and continues to the discharge area. Again, metering of the release agent onto the fuser rolls is important because too much release agent soils or otherwise degrades the quality of the media and the image on it.
In one known toner printer, an optical system for detecting oil on a fuser roll is known. The optical system directs a light onto the fuser roll and the amount of reflected light is used to determine whether or not there is a sufficient amount of oil on the fuser roll. Such an optical system, however, cannot be used on a print drum in an ink jet or solid ink printer to measure release agent on an image member. One reason that the optical system of the toner printer cannot be used to measure release agent on a print drum arises from the differences in surface textures and base reflectivity between print drums and fuser rolls. Print drums have anodized and etched surfaces. Consequently, the voids in the surface of print drums are microscopic and the base reflectivity is relatively high compared to fuser rolls. These characteristics render known optical sensing systems ineffective for measuring the amount of release agent on the surface of the drum.
Light sensing systems are used in toner printers to evaluate the developed mass of toner applied to a photoreceptor. These systems include a light source and a light sensor. In one typical system, an ETAC sensor is configured and positioned to direct a collimated beam of light towards a photoreceptor, which reflects the light towards a plurality of photodetectors. This configuration enables the sensor to detect differences between a toned patch and the untoned photoreceptor. The light source generates a collimated light beam having a fairly constant diameter. The light also has a dominant wavelength that may be in the visible or infrared spectrum. For example, one ETAC sensor uses a light source that generates a light beam approximately four millimeters wide with a wavelength of approximately 940 nanometers.
An ETAC sensor may be used to detect release agent on a fuser roll in a toner printer. The base reflectivity combined with the pores and ridges in a fuser roll are sufficient to absorb and scatter the incident beam to allow a small amount of the incident light to be returned to the sensor's detectors in an un-oiled state. As the fuser roll surface fills with oil, the irregularities are reduced, the surface becomes smoother, and more of the light is returned to the sensor, rather than being scattered. Thus, the greater the amount of oil on the surface of the fuser roll, the greater the intensity of the light at the sensor.
The base reflectivity of an anodized and etched print drum, however, is so high in comparison to a fuser roll, that the light reflects off of the bare surface of the print drum in practically the same manner as it does when the surface is covered with release agent. That is, the field of view and detection methods are inadequate to separate the changes in surface characteristics as the amount of oil applied to the surface varies.
While a light detection and measurement system for release agent is desirable, because it does not mechanically disturb the oil layer, known systems are ineffective for detecting and measuring release agent on the substantially smoother image members of ink jet printers. A system for detecting and measuring release agent on a print drum and the like would be an improvement over the level sensing sensors currently used in such printers.